Phase locked loops (PLLs) are widely used to provide clock signals for integrated circuits in radio, telecommunications, and other applications where a stabilized frequency source or detection of a signal in noise is required. For example, in wireless technology, systems with multiple frequencies are in use, with frequencies in a single system ranging from several hundred megahertz up to a few gigahertz. For compact and power-efficient systems supporting multiple wireless standards, maximum hardware sharing is necessary.
PLLs are closed-loop feedback systems that generate a signal equal in phase and frequency in relation to an input signal. Within the feedback loop of the PLL is a voltage-controlled oscillator (VCO), which generates a signal at a frequency that is a function of the applied bias. Typical designs for VCOs include LC-tank oscillators, crystal oscillators, surface acoustic wave oscillators, and ring oscillators. Of these types of oscillators, only LC-tank oscillators and ring oscillators lend themselves to integration in standard CMOS designs. In general, for a given design, these oscillators have a narrow tuning range, which would dictate that there be multiple clock sources for applications requiring a variety of clock frequencies.
While LC-tank oscillators are capable of accurate clock signals, they generally require an off-chip inductor or an on-chip spiral inductor. Integrating a high quality inductor into a standard CMOS process is not trivial, being limited by parasitic effects and the complexity of added non-standard process steps.
As integrated circuit processes have moved to smaller dimensions and lower supply voltage, inverter-based CMOS-based ring oscillators have become increasingly attractive. In voltage-controlled oscillators, the frequency is tuned by adjusting the supply voltage, which results in frequencies in a specific tuning range. However, when frequencies are required outside of this tuning range, multiple different oscillator designs must be utilized. Designing a different oscillator for each application is extremely costly and time consuming.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.